Signal frequency inverter



March 8, 1938. e. M. CHARRIER 2,110,698

4 SIGNAL FREQUENCY INVERTER Filed Oct. 13, 1936 2 Sheets-Sheet 1 I 3nventor v qye faarr" attorney March s, 1938. I GM HARRIER 2,110,698

SIGNAL FREQUENCY INVERTER Filed Oct. 15, 1936 2 Sheets-Sheet 2 3twentor 6507' a Tic? attorney UNITED STATES PATENT OFFICE SIGNAL FREQUENCY INVERTER George M. Charrier, Oaklyn, N. .L, assignor to Radio Corporation of America, a corporation of Delaware Application October 13, 1936, Serial N... 105,363

6 Claims. (cans-4.5)

My invention relates to communication systems ample, the filter l may pass currents of from and more particularly to means for inverting the 300 to 3000 cycles per second for the transmission signal frequency at the transmitter and means of speech. for reinverting said inverted signals at the re- The signal frequency inverter corprises a pair 5 ceiver. of multi-purpose thermionic tubes l1, l9. Two 5 I am aware that radio systems for secret comof the grids 2 I, 23 of these tubes are connected to munication have been based on frequency ina tuned circuit 25, and through resistors 21, 29' version. The term frequency inversion has been to the cathodes 3|, 33. In many cases a single reused to describe the conversion of low signal fresistor from. the midpoint of the inductor 35 to quencies into high frequencies and high signal ground may-be used in place of the pair of re- 10 frequencies into low signal frequencies. Speech sistors 21,29. The cathodes 3|, 33 are connected frequencies which are thus inverted convey subto ground by self-bias resistors 35, 31. One of stantially no intelligence until such inverted sigthese resistors 35 is made adjustable to balance innals are reinverted. equalities in the tube characteristics. The self- 18 One of the objects of my invention is to provide bias resistor 35, 31 are repectively bypassed by means for inverting currents of signal frequencies capacitors 39, 4! having, by way of example, a at a transmitter and means for reinvertlng such capacity of 5 microfarads. The tuned grid cir inverted signals at a receiver. cult 25 is coupled to an anode circuit inductor 43,

Another object is to provide a simple, compact the terminals of which are joined to the grid anode 20 means for the inversion of signal frequencies electrodes 45, 41. The anode circuit is connected, 20 which may be readily attached to existing transthrough resistors 48 or a midtap, to the positive mitters and receivers. terminal 49 of a B battery source 5|.

A further object is to provide means for feed- The balanced modulator portion of the signal ing back currents of undesired frequencies from frequency inverter comprises control grids 53,

the output to the input of a signal frequency in- 55 which are connected in parallel and to ground 35 verter to oppose the transfer of such undesired through a resistor 51. The output of the input currents. filter I5 is connected directly to these control A still further object is to connect a pair of grids. The controlgrids53, 55 are shielded from multi-purpose thermionic tubes so that sections of the oscillator electrodes 2|, 23, 45, 41 and the outeach of said tubes act as push pull oscillators and put anodes 50, 6| by screen grids 63, 65 which are a other sections act as balanced modulators. biased positively with respect to the cathodes 3|,

' One embodiment of my invention has been 11- 33 by a connection to the potentiometer 6'! which lustrated in Fig. 1 which is a schematic circuit is connected across the B battery 5|. The outdiagram of a signal frequency inverter applied to put anodes 59, ii are connected to the primary 35 atransmitter, and 63 of the push pull or balanced output trans- 5 Fig. 2 is a schematic circuit diagram of a signal former 69. The midpoint of the primary 0! is frequency inverter applied to a receiver. connected to the positive terminal 49 of the 3 Referring to Fig. 1, an oscillator I, which genbattery. erates currents of carrier frequency, is coupled The secondary ll of the. output transformer to a power amplifier 3 which is in turn coupled is connected to an output filter 13 which is 40 to an antenna 5. A microphone I is connected to designed to pass the desired currents of inverted an audio amplifier 9. If the switch II were signal frequency and attenuate the undesired curclosed, the audio amplifier would be connected to rents which are created by such inversion. At a modulator l3, whose output is impressed on speech frequencies it is both diflicult and expenthe power amplifier 3 to thereby modulate the sive to provide a filter having sufficiently sharp 45 carrier currents. The foregoing system is that of cut-01f to eliminate currents of the oscillatory a conventional radio transmitter. frequency and the sum frequency. I have found If the switch II is opened as shown, the output that the high potential terminal 15 of the outcurrents from the audio amplifier 9 are impressed put of the filter 13 may be connected through across the input of the signal frequency inverter. a capacitor 11 to the slider of a high resistance 50 The inverter input circuit includes an input filter potentiometer I! connected between the cathodes l5 which passes currents of the useful range of 3|, 33. Although the impedance of the self-bias speech or signal frequencies to prevent audio resistors and bypass capacitors is low, it is just flutter and interference with the useful signal suflicient to permit the required feedback. By

as frequency range of the receiver. By way of exasuitable adjustmentthisconnection maybeused to impress currents which oppose the currents of undesired frequency which are not sufliciently attenuated in the filter I3. The output from the filter I3 is impressed on the modulator I3.

The operation of the inverter is essentially as follows: The push pull oscillator connection generates oscillatory currents whose frequency primarily depends upon the constants of the tuned circuit 25. I prefer to use a large value of capacity to insure stable oscillations, preferably higher than the signal frequencies, and for speech, of the order of 4000 cycles per second. The speech or audio frequency currents are impressed on the control grids 53, 55. Since the output of tubes I'I, I9 is arranged as a push pull or balanced circuit connection and the input as a parallel connection, the audio frequency input currents will balance out in the balanced output transformer 69. The oscillator section being arranged as a push pull input and output will transfer currents of the oscillator frequency to the output transformer 69. However, the audio frequency currents and the oscillator currents will interact on each other and form component currents of a frequency equal to the sum and difference frequencies. By way of example, a few frequencies are represented in the accompanying table:

If currents of the sum frequency and the oscillator frequency are attenuated by the output filter I3, and currents of the difference frequency are passed by the output filter I3, it is apparent that the high input frequencies become the low output frequencies and vice versa. For example, 300 cycles per second becomes 3700 cycles per second and 3700 cycles input produces a 300 cycle output. In brief, the frequencies are inverted. It will be observed that the oscillator frequency is higher than the upper limit of the signal frequency.

The carrier currents, modulated by currents of inverted signal frequency, may be received by a conventional radio receiver and demodulated but the demodulated signal currents produce audio signals which are unintelligible because of the frequency inversion. These demodulated currents may b inverted to reproduce the original signals. The inverter at the receiver is essentially the same as the signal inverter at the transmitter. In Fig. 2, an antenna IN is suitably coupled to a radio frequency amplifier I03 which may be tuned to carrier frequency. The output circuit of the amplifier I03 is coupled to a detector I05 which is in turn connected to an audio frequency amplifier I'I. If the double throw double pole switch I09 is placed in the upper position III, the audio frequency amplifier will be connected to a second audio amplifier I I3 and a loudspeaker H or the like.

If, however, the switch I 09 is placed in the lower position, the output of the audio amplifier I0I is connected to the input or control grids of the inverter. This inverter is a duplicate of the inverter previously described. In view of this similarity, the inverter at the receiver will not be described but the same reference numerals will be used to indicate parts which are similar to the inverter at the transmitter.

It may be observed that the input filter I5 is omitted from the inverter employed at the receiver. The receiver is only responsive to the range of frequencies transmitted and therefore requires no further limitation. The output filter is, of course, necessary to attenuate currents of the sum frequency and oscillator frequency. As in the case of the filter at the transmitter, the attenuation is greatly improved at frequencies of the order of the local oscillator frequency by the feedback connection through capacitor 11 and potentiometer I9 previously described. The frequency of the local oscillator of both transmitter and receiver inverters is preferably the same. This avoids any shift of the whole signal frequency band which would result from different inverter oscillator frequencies.

Thus I have described a communication system in which the normal modulating signal currents are fed through a signal frequency inverter. By a suitable balanced system the original signal frequencies are eliminated from the output of the inverter. Undesired currents, which have a frequency equal to the sum of the inverter oscillator and the signal frequencies, are attenuated by an output filter. Residue currents of undesired frequency such as the frequency of the inverter oscillator, are fed back from the output of the filter to the local oscillator input to thereby oppose the currents of undesired frequency. A similar inverter at the receiver restores the received demodulated currents to the original signal frequency. While the foregoing system has been described in connection with a speed modulated transmitter, it should be understood that the frequency inversion may be applied to facsimile, television and the like.

I claim as my invention:

1. A signal frequency inverter comprising a source of signal frequency currents, means for generating local oscillatory currents of a frequency greater than said signal frequency, means for combining said local currents and said signal currents to establish currents having a frequency equal to the sum and difference frequencies of said local and signal frequencies, and means for attenuating currents of said sum frequency and said local oscillator frequency.

2. A signal frequency inverter comprising a source of signal frequency currents, a pair of thermionic tubes including electrodes connected in push pull arrangement for the generation of local oscillatory currentsof a frequency greater than said signal frequency, means for impressing said signal frequency currents on parallel connected control electrodes in said tubes, a balanced output circuit, means connecting said output circuit to the outputs of said tubes, whereby said signal frequency currents are balanced in said output transformer and said signal currents and oscillator currents interact to form components whose frequencies equal the sum and difference frequencies of said signal and oscillatory currents, and means for attenuating currents of said sum frequency and for passing currents of said difference frequency to a utilization circuit.

3. In a device of the character of claim 2, means for impressing attenuated output currents on the input of said inverter to further attenuate currents of undesired frequencies.

4. A signal frequency inverter comprising a source of current of signal frequencies, a source of local oscillator currents of a frequency different from said signal frequencies, means for impressing said signal frequency currents on a balanced output circuit, means for combining said signal currents and said local currents to create component currents in said output circuit equal to said oscillatory frequency and the sum and difference of said local oscillatory frequency and said signal frequencies, means for attenuating currents of said oscillator frequency and said sum frequency, and a feedback circuit for feeding back currents to oppose currents of said local oscillatory frequency.

5. A signal frequency inverter comprising a source of signal frequency currents, a pair of multi-purpose thermionic tubes each having cathode, grid, grid-anode, control grid and anode electrodes, a push pull oscillator circuit connected to said cathode, grid and grid-anode electrodes,

means for connecting said control grids in parallel, means for impressing said signal currents on said control grids whereby currents equal to said signal frequency, and oscillator frequency, and the sum and difference of said frequencies are created, a balanced output circuit connected to said anode electrodes, means for attenuating currents of said oscillator frequency and said sum frequency and transferring currents of said difference frequency fro-m said balanced circuit to a utilization circuit, and means for adjusting the characteristics of said tubes whereby their characteristics may be substantially equalized.

6. In a device of the character of claim 5 feedback means for connecting said attenuation means to said oscillator to oppose said currents of undesired frequency.

GEORGE M. CHARRIER. 

